1. Field of the Invention
The present invention relates to a polymer suitable as a base resin for a positive resist composition, in particular a chemically amplified positive resist composition, to a positive resist composition using the polymer, and to a patterning process.
2. Description of the Related Art
As LSI progresses toward a high integration and a further acceleration in speed, a finer pattern rule is rapidly progressing. In particular, the expansion of a flash memory market and the enlargement of a memory capacity lead the finer pattern rule. Mass production of a 65-nm node device by a miniaturized fine line of an ArF lithography is currently in practice and preparation of the mass production of a 45-nm node device by a next generation ArF-immersion lithography is now progressing. As the candidates for the post next generation technology, a 32-nm node, an immersion lithography by a ultra high NA lens comprising a liquid having a higher refractive index than water, a high-refractive lens and a high-refractive resist, an extreme ultraviolet (EUV) lithography of 13.5-nm wavelength, and a double exposure (a double patterning lithography) of an ArF lithography, and the like are being studied.
In a high energy beam of an extremely short wave length such as an EB, a X-ray, and the like, a light element such as a hydrocarbon used in a resist has almost no absorption, and thus a resist composition based on polyhydroxystyrene is studied.
An EB resist has been practically used for a mask lithography. In recent years, a mask production technology has been considered to be a problem. A reduced projection exposure system with a 1/5 reduction ratio had been used from the time of a g-beam exposure, but an effect of the size misalignment of a mask on a size change of a pattern on a wafer becomes a problem since an enlarged chip size, a projection lens with an enlarged diameter, and also a 1/4 reduction ratio have been employed. In addition, as the pattern miniaturization progresses, it is pointed out that a size misalignment on a wafer is becoming larger than a size misalignment of a mask. A Mask Error Enhancement Factor (MEEF) is calculated by using a mask size change as a dominator and a size change on a wafer as a numerator. In the pattern of a 45-nm class, it is not a rare case that MEEF is over 4. If the reduction ratio is 1/4 and MEEF is 4, it can be said that the equivalent precision as a substantially same magnitude mask is necessary in the mask production.
In order to improve a precision of a line width in a mask production, exposure systems using a laser beam to an electronic beam (EB) have been used. In addition, as a further miniaturization becomes possible by raising an acceleration voltage of an electron gun, a voltage of 10 to 30 keV, and of 50 keV becomes a mainstream recently, and further the study is progressing toward 100 keV.
Here, as an increase of the acceleration voltage, a tendency to a lower sensitivity of a resist becomes a problem. A higher acceleration voltage enables to improve the degree of resolution and a size controlling as the effect of a front scattering in a resist film becomes smaller, leading to an improvement in a contrast of an electronic lithography energy, but a sensitivity of a resist is decreased as an electron passes through a resist film freely. Because a mask exposure system exposes a light directly for a lithography by a traversable line, the decrease of the resist sensitivity leads to a decrease in productivity and thus is not desirable. In view of a request for a higher sensitivity, a chemically amplified positive resist composition is being studied.
As a miniaturization of a pattern of an EB lithography for a mask production progresses, a move to a thinner resist is progressing in order to prevent a pattern fall due to a high aspect ratio at the time of development. In the case of a photolithography, the move to a thinner resist contributes greatly to the improvement in a resolution. This is owing to a flattening of a device by introduction of a CMP and the like. In the mask production, a substrate is flat, thus a film thickness of the substrate to be processed (for instance, Cr, MoSi and SiO2) is determined for the light shielding rate and the phase difference control. In order to make it thinner, it is necessary to improve a dry-etching resistance of a resist.
Meanwhile, it is generally assumed that there is a relationship between a resist carbon density and a dry-etching resistance. In the EB lithography not affected by an absorption, a resist based on a novolak polymer having a good etching resistance has been developed. However, a novolak polymer is not assumed to be a suitable composition for a fine processing as it is difficult to control a molecular weight and a degree of dispersion.
In addition, it is reported that an absorption of a carbon atom is small in a soft X-ray (EUV) exposure using a 5-20 nm wavelength, which is expected, along with an F2 exposure, as a promising exposure method in a fine processing using a 70 nm wavelength or in a process that follows thereafter. A higher carbon density is effective not only for improvement in the dry-etching resistance but also for increase in a transmittance in the region of a soft X-ray wavelength (N. Matsuzawa et. al.; Jp. J. Appl. Phys., Vol. 38, p 7109-7113 (1999)).